I use chargers which are designed to take a low voltage dc input, via an external power supply.
I have two chargers, but my favourite is this one- http://www.hobbyking.com/hobbyking/stor ... arger.html
Extremely flexible in operation, it charges pretty much every battery tehcnology on the market. I run this from a 12 volt 5A switch-mode power supply, which is ex-equipment from a HP computer. I just snip off the manufacturers connector and wire on my own.
There are cheaper chargers available, and I must admit the choice is somewhat bewildering.